Pulse-oximetry is a non-invasive method that may be used to monitor the saturation of a patient's hemoglobin. Pulse-oximetry typically utilizes a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body, usually a fingertip or an earlobe. One LED is red, with wavelength of 660 nm, and the other is infrared, 905, 910, or 940 nm. Absorption at these wavelengths differs significantly between oxyhemoglobin and its deoxygenated form; therefore, the oxy/deoxyhemoglobin ratio can be calculated from the ratio of the absorption of the red and infrared light. The monitored signal bounces in time with the heart beat because the arterial blood vessels expand and contract with each heartbeat. By examining only the varying part of the absorption spectrum (essentially, subtracting minimum absorption from peak absorption), a monitor can ignore other tissues or nail polish, and discern only the absorption caused by arterial blood.
In measurements systems such as for pulse-oximetry, the desired signal has a very small amplitude that rides on a large ambient offset signal. In pulsed input measurement systems, such as for oximetry, the front end transimpedance amplifiers or gain amplifiers need to have a large bandwidth in order to support the pulsing input signals. The equivalent noise bandwidth of the front end is larger because of the large bandwidth for settling, even though the signal bandwidth of interest is much smaller. The front end gain is usually restricted to avoid saturation of the front end due to the large offset signal in comparison to the signal of interest. Since the front end needs to support large signal swings because of the offset, it is typically operated on a higher supply voltage which increases power consumption.
Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description that follows.